Conventional color laser printers known as tandem-type printers are provided with process cartridges corresponding to each of the colors yellow, magenta, cyan, and black that are juxtaposed along a path for conveying paper.
This type of tandem color laser printer has, for example, a main device body with a cover provided on the top surface thereof that is capable of opening and closing. By opening the cover, the process cartridges can be mounted into or removed from the main device body. A motor is also provided in the main device body for generating a driving force that is transmitted to each process cartridge when the process cartridges are mounted in the main device body. The driving force drives a photosensitive drum and a developing roller provided in each of the process cartridges in order to form a toner image in each color on the respective photosensitive drums nearly simultaneously. These toner images are transferred onto a paper conveyed along the paper-conveying path in order to form a multicolor image on the paper at approximately the same speed required to form a single color image with a monochrome laser printer.
Japanese unexamined patent application publication No. HEI-11-258966 describes one mechanism for transmitting the driving force of the motor to each photosensitive drum. This mechanism includes a drum drive shaft provided in the main device body for each photosensitive drum, the drum drive shaft capable of being shifted to advance or retract; a compression spring for urging the drum drive shaft in the retracting direction away from the photosensitive drum; a lever cam having one end rotatably supported about a rotational shaft and another end contacting an end of the drum drive shaft on the side opposite the photosensitive drum; and a vertical lever that is moved vertically when a top cover member is opened and closed on the top surface of the main device body. When the top cover member is closed, each vertical lever moves downward, the lower end of each lever pushing against and rotating the respective lever cam. As the lever cams rotate, each drum drive shaft advances and engages with the respective photosensitive drum.
However, providing the same mechanism that includes the drum drive shaft, lever cam, and vertical lever for each photosensitive drum will inevitably increase the size and expense of the image-forming device due to the large number of parts.